Implantable cardiac devices include devices such as pacemakers and implantable cardioverter-defibrillators (ICD's). These are devices which are configured to be implanted within the body of the patient and have leads that are adapted to be positioned adjacent the walls of the heart that can provide therapeutic electric electrical stimuli to the heart. Over time, implantable cardiac devices have become more sophisticated. The current generation of implantable cardiac devices are capable of sensing the occurrence of a particular arrhythmia and then providing an appropriately configured therapeutic electrical shock or stimulus.
For example, an implantable cardioverter-defibrillator will be able to sense the occurrence of a ventricular fibrillation episode and provide a defibrillation shock to the heart to restore normal rhythm. Similarly, current generation pacemakers are capable of providing pacing pulses to the heart upon detecting an absence of appropriate intrinsic activity of the heart.
These implantable cardiac devices are also equipped with sensing circuits that provide an indication as to the intrinsic activity of the heart. These sensing circuits provide the input which allows the implantable cardiac device to selectively apply the appropriately configured therapeutic electrical stimuli. Typically, these implantable cardiac devices can continuously monitor the heart and store a plurality of diagnostic signals contemporaneously with therapy. The recorded data can then be subsequently downloaded, via a telemetry system, to an external device, thereby allowing a physician to observe the event which triggered the therapeutic action at a later time.
Being able to review a record of the heart's activity during a cardiac event necessitating the application of a stimulation therapy is a very valuable diagnostic tool for the physician. For example, repeated occurrence of a particular cardiac event may be indicative of an additional problem suffered by the patient that would necessitate additional treatment.
It allows configuration of the device to give the best therapy, and provides information about the progress of the disease that can guide prescription of the treatment.
However, one difficulty with current implantable cardiac devices is that the recording capability is often limited. For example, a device that has 128 Kbytes of memory and samples the electrogram signal with 8-bit resolution at a rate of 256 samples/second can store only approximately eight minutes of electrograms. As patients may visit their physicians only periodically, eight minutes of recording time may be insufficient to capture an electrogram of a significant number of the episodes that occurred during the interval between the patient's visits to the doctor.
Hence, certain events are either not recorded or, if they are recorded, they are overwritten by more recent events. This loss of data can impede the ability of the physician assess the patient's condition.
Hence, there is a need to increase the memory capacity of the implantable cardiac device so that more data can be recorded. However, the design constraints of implantable cardiac devices impose limits on the amount of memory which can be included in the device. In particular, the controller for implantable cardiac devices are generally very small in size and space for memory is generally at a premium. Further, implantable cardiac devices are also battery operated and the addition of processing circuitry to increase memory capability may result in a decrease in the active life of the implantable cardiac device. Battery depletion will necessitate replacement, which requires an invasive surgical procedure. Therefore, a decrease in the active life of the implantable cardiac devices to improve memory capabilities is generally undesirable.
Data compression is one way of increasing the amount of data that can be stored in a memory, without increasing the size of the memory. Compression generally means storing the data in such a manner that it requires less memory space. Subsequently, when the stored data is downloaded, the compressed data can then be reconstructed into its original form.
There are generally two types of compression schemes, lossy and lossless. Lossy compression schemes yield a higher compression ratio than lossless schemes, at a cost of some loss in data following reconstruction. A lossy compression scheme is a compression scheme which stores the original data in such a fashion that when reconstructed, the reconstruction data is not an exact replication of the original data. Generally, lossy schemes determine that some portion of the data is less important and will therefore not store as precisely these portions of the data.
Lossless schemes compress the data in a manner which allow the data to be reconstructed such that the reconstructed data is the same as the original data.
While compression schemes are commonly used in applications such as digital communications, they have generally not been used in implantable cardiac devices. One reason for this is that compression schemes often require significant processing. The processing requirements to implement most compression schemes generally have a negative impact on the battery life of an implantable cardiac device. Further, prior art efforts to implement compression schemes to compress electrograms in implantable cardiac devices have often necessitated the inclusion of additional components into the implantable cardiac device. These additional components take up valuable space in the implantable device and consume additional power from the batteries.
One example of such a prior art application of compression schemes to implantable cardiac devices is provided by U.S. Pat. No. 4,716,903 to Hanson et al. which discloses a pacemaker memory that has a compression circuit which allows for an electrogram signal to be compressed. In particular, U.S. Pat. No. 4,716,903 compresses a digital conversion of an electrogram signal by recording the positive or negative changes in the signal and also recording the elapsed time between each change. This compression scheme requires the addition of registers, adders and a clock to implement the compression scheme. Each of these components take up additional space in the limited space environment of the pacemaker controller. Further, each of these additional components must be powered by the battery and therefore diminish the active life of the battery and of the implantable cardiac device.
Hence, there is a need for an implantable cardiac device which has enhanced recording capability that does not require a significant number of additional components or result in significant consumption of battery power. To this end, there is a need for an implantable cardiac device which has the ability to compress event data in a manner that is efficient in terms of power consumption and also in terms of consumption of limited space within the control unit of the implantable device.